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Leprosy is an ancient disease; sacred writings from India in the sixth century bc give a good description of a similar or identical illness. By the second century ad, Greek physicians in Europe wrote descriptions of the disease, evidently well established by that time. Stigmatization of patients with leprosy remains an unfortunate but enduring legacy of the European pandemic that occurred from 1,000–1,500 ad Hansen's attribution of Mycobacterium leprae as its etiologic agent in 1873 marks the beginning of scientific leprology.1

Effective chemotherapy began with the introduction of sulfones in 1943. In 1961, the limited growth of M. leprae in the mouse footpad provided a way to screen for therapeutic agents and to identify drug resistance.2 In 1970, rifampin was the first drug to be identified as bactericidal for M. leprae and is now the cornerstone of most therapeutic regimens. Ridley and his associates3 provided a detailed account of the granulomatous spectrum of leprosy that is useful to clinicians and pathologists and essential to subsequent immunologic studies. Jopling clearly defined and separated the two common reactional states.

In 1919, the lepromin skin test inaugurated systematic study of host resistance as the source of disease diversity,4 with lymphocyte transformation tests being established in the 1960s as an in vitro correlate. The recognition of leprosy in the nine-banded armadillo, in 1971, provided a source of large quantities of highly purified M. leprae for a wide variety of investigations,5 culminating in the sequencing of the entire genome of M. leprae in 2001.6

Now primarily a disease of developing countries, leprosy is endemic in all continents, except Antarctica. In the Americas, only Canada and Chile are not endemic countries, with Texas and Louisiana having endemic foci in the United States. The southern-most nations of Europe have a very low incidence, while leprosy is endemic in many Pacific islands. The Indian subcontinent has two-thirds of the world's leprosy burden. The highest case detection rates are in Angola, Brazil, Central African Republic, Democratic Republic of the Congo, India, Madagascar, Nepal, and Tanzania.7,8 During the decade of the 1990s, the prevalence of leprosy fell by 90%, because patients completing a course of multiple-drug therapy have been considered cured, but the incidence of the disease, which varies in direct proportion to case finding efforts, has not been convincingly reduced.